Composite structures are widely used to build up whole blades or even components for a blade of a wind-turbine. For this a matrix of fiber glass mats is used. The building-up of the matrix is characterized by a considerably amount of manual layup work, where fiber mats with fibers of different orientation are put on top of each other in order to build up the strength and stiffness of the blade. The manual layup work is difficult and time-expensive.
Fiber mats are standard components and can be handled easily in a manual layup work. Fiber mats or sheets of fibers are made of woven and non woven fabric. The nonwoven fabrics are made of unidirectional fibers held together by a chemical binder or the like. The woven fabrics are made on looms where fibers pass over and under each other in a bidirectional manner in order to bind the fibers together.
A beam of a blade of a wind-turbine comprises a number of layers of fabrics having unidirectional fibers. The fibers are aligned in a longitudinal direction of the blade in order to give the correct strength and stiffness in a flapwise direction of the blade.
A so called “multi-axial mat” (such as a biaxial mat) is placed on top to ensure a torsion-flexibility and strength to the blade structure. The number of layers and directions of the fibers can be changed to achieve different mechanical properties at different positions of the blade.
The structural characteristics of a fiber reinforced laminate are usually governed by the amount, type and orientation of the reinforcement fibers. Typically, the stiffness and strength of fibers can only be taken into account to the extent that loading occurs in the longitudinal fiber direction.
Therefore, a traditionally designed laminate assumes that the fibers of the finished laminate will be oriented in the same direction as the direction of the fibers when placed in a forming tool such as a mould for a wind-turbine blade.
However, in many cases wrinkles in the fiber layers occur as a result of the manufacturing process and the characteristics of the fiber mats and fabrics used. Wrinkles are likely to develop in the manual layup work due to the woven characteristics or the interconnection of the fibers in the non woven fabrics. Highly skilled and experienced layup workers are needed to ensure that wrinkles are not introduced into the blade-structure during the difficult layup work.
However, despite the experience of the layup workers wrinkles may occur. When wrinkles occur the fibers no longer have the desired orientation and severe overload of the laminate may be the result.
Repair or rejection of the laminate will usually be required if wrinkles do occur, as the loss of stiffness and/or strength in wrinkles will often exceed any realistic safety margins of the structure. The repair work is done by hand and is therefore tedious and costly. Rejection of the entire structure is even more costly and needs to be avoided.
The thickness of the laminate needs to vary in order to obtain the correct structural characteristics and the desired aerodynamically shape of the blade structure. The stiffness of the wind-turbine blade of course depends on the shell thickness, the cross-sectional geometry and the material.
The cross-sectional dimensions of the wind-turbine blade and the thickness vary in the longitudinal direction of the blade. The variation in the thickness is controlled by reducing or increasing the number of plies of laminates in the structure.
So called “ply drops” are formed in areas, where the thickness changes. At these positions one or more layers are terminated, so discontinuities are introduced in the material and in the geometry of the structure. Discontinuities induce stress concentrations at the ply drop areas. In many cases these areas are decisive for the life time and the strength of the whole structure.
The geometrical discontinuity of the ply drop zone leads to an accumulation of resin in so called pockets during a curing process. The resin-rich pockets could lead to a development of cracks in the structure.
A lot of research work needs to be spent to achieve an optimum design in regard to the ply drop zones. However, it is very difficult to implement optimized ply drop zones if a manual layup work is used.
The fiber mats used are typically large and heavy and difficult to handle when they are positioned in a forming tool; a precise layup and alignment of the fiber mats is very difficulty.